1. Field of the Invention
The present invention relates to a throttle valve control system for controlling the opening angle of a throttle valve of an internal combustion engine.
2. Description of Background Information
A prior art throttle valve control system is described, for example, in Japanese Laid-open Patent No. 60-192843, which describes an apparatus whereby a throttle valve is driven to an opening angle which is determined in accordance with the operating position of the accelerator and the engine rotational speed, for providing improved engine response. Means have also been envisaged for controlling the throttle valve opening angle such as to reduce the engine fuel consumption, for example by deriving a minimum fuel consumption factor in accordance with the engine rotational speed, setting a target value for the pressure within the intake pipe of the engine (referred to in the following as the induction manifold pressure), and driving the throttle valve such as to reduce the amount of deviation between the actual induction manifold pressure and the target induction manifold pressure. Such a system has been proposed hitherto by the assignees of the present invention.
However with a throttle valve control system whereby the throttle valve is controlled such as to make the actual induction manifold pressure become equal to a target inlet manifold pressure, there is a tendency for the throttle valve to be driven to an excessively high opening angle when the engine is used in a region at a high altitude, i.e. is operated under a lower atmospheric pressure than normal. As a result, if the throttle valve is abruptly controlled such as to move in the closing direction, there will be a substantial amount of control delay. This control delay is introduced as a result of the excessively open condition of the throttle valve. Thus, satisfactory engine operating response may not be obtained.
Furthermore usually with such a throttle valve control system, the more deeply the accelerator pedal is depressed, the greater will be the amount of control which is applied to the throttle valve opening angle. However since the range through which the throttle valve can be driven by such control extends from a condition of being fully closed to that of being fully opened, a large degree of control overshoot can occur when the throttle valve is driven in accordance with the amount of deviation between a detected value and a target value. An excessive amount of throttle valve opening can thereby result, which produces a delay in response to such control and can cause fluctuations in the engine output power. Satisfactory engine operation may therefore not be attainable.